This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
The present disclosure relates generally to a steering assembly for directionally drilling a borehole in an earth formation, and more particularly to a steering assembly comprising an under-gauge section and an over-gauge section and configured for disposal above a drill bit.
Directional drilling is the intentional deviation of a borehole from the path it would naturally take, which may include the steering of a drill string so that it travels in a predetermined direction.
In many industries, it may be desirable to directionally drill a borehole through an earth formation in order to for example circumvent an obstacle and/or to reach a predetermined location in a rock formation.
In the oil and gas industry, controlled directional drilling began in the mid-20th century as a technique to reach otherwise inaccessible hydrocarbon reserves. Early directional drilling involved the use of deflection or side-tracking devices such as whipstocks and simple rotary assemblies to reach the desired target. However, this approach was time-consuming, involving multiple trips of tools and pipe into and out of the borehole, and offered limited control, frequently resulting in missing the target.
Introduction of positive displacement motors offered steering capability and, with it, some degree of directional control. However, these motors lacked the efficiency drillers sought, mainly because of the slide drilling involved.
Slide drilling refers to drilling with a mud motor rotating the bit downhole without rotating the drill string from the surface. The bottom hole assembly at the lower end of a drill string is fitted above the bit with a bent sub or a bent housing mud motor, or both, for directional drilling. With such systems, the bent sub and the bit are pointed in the desired direction. Without turning the drill string, the bit is rotated with a mud motor, and slides in the direction it points. When the desired wellbore direction is attained, the entire drill string is rotated and drills straight rather than at an angle. By controlling the amount of hole drilled in the sliding versus the rotating mode, the wellbore trajectory can be controlled.
Positive displacement motors can produce extreme torque and drag that can limit drilling capability in sliding and rotating modes. Steerable motors can produce unacceptable wellbore tortuosity when drilling in the rotating mode, making further sliding more difficult and impeding critical operations for formation evaluation and running casing. Rotary steerable systems (RSS), which drill directionally with continuous rotation from the surface while pushing the bit or pointing the bit towards the target direction, were introduced to address these issues. RSS eliminate the need to slide the drill string; through continuous rotation transfer weight to the bit more efficiently, thereby increasing rate of penetration; improve hole cleaning by agitating drilling fluid and cuttings, thereby allowing cuttings to flow out of the hole rather than accumulating in cuttings beds; improve directional control in three dimensions; and with a smoother and cleaner wellbore, make formation evaluation and running casing less complicated with reduced risk of getting stuck. However, RSS perform via surface rotation, making them rig-dependent, offer limited selection of bit sizes and speeds, and involve increased mechanical and electronic complexities that can lead to increased costs.
Known forms of RSS include a “counter rotating” mechanism which rotates in the opposite direction of the drill string rotation. Typically, the counter rotation occurs at the same speed as the drill string rotation so that the counter rotating section maintains the same angular position relative to the inside of the borehole. Because the counter rotating section does not rotate with respect to the borehole, it is often called “geostationary” by those skilled in the art. For example, U.S. Pat. No. 8,727,036 is directed toward a geostationary steering cylinder comprising a first under-gauge or full-gauge peripheral section and a second peripheral section opposing the first section, where the distances from the two sections to the center of the bit differ by between 0.5 mm and 20 mm. In particular, FIG. 8K of U.S. Pat. No. 8,727,036 discloses a steering cylinder with a profile which is circular and offset from the drill bit. However, in practice this configuration is difficult to manufacture with precision due to the very small displacement needed and has an under-gauge section that is likely to block the steering cylinder from drilling forward.